A Study of Novel Aluminium Alloys for Thermally Sprayed Metallic Coatings

Aluminium possesses a standard electrode potential which is more negative than that of steel (-1.66 V vs. NHE), which makes it potentially suitable as an anode for cathodic protection systems[1,2]. However, when aluminium is exposed to air or aqueous environments, the formation of a passive oxide layer on the surface inhibits the anodic properties of aluminium[1,3]. In order to maintain an active surface, aluminium can be alloyed with various elements such as Zn, In, Mg, Ti and others, which maintain an active surface by preventing the formation of a passive oxide layer, thus enhancing its ability to galvanically protect[4]. Hence aluminium alloy coatings act not only as a physical barrier between the substrate and the environment, but may also protect the substrate sacrificially[5].Thermally sprayed aluminium alloy coatings are widely used for the corrosion protection of steel substrates[6]. The process has the ability to deposit thick coatings, ranging from 40 to 500 µm, deposit a diverse range of materials and the ability to perform coating repairs in-situ[7]. The main objective of the current work was to investigate the performance of commercially available Al and Zn-Al coatings and compare these to a range of new Al-Zn-In coatings which are being developed for use on steel substrates. In order to evaluate the coating performance, the coatings were exposed to a 5% w/v NaCl environment for 1000 h in an ASTM B117 salt spray corrosion test. Masked and unmasked coating samples were used during the test for visual and corrosion product examination. The corrosion product obtained after 500 and 1000 hours test run was analysed using X-Ray Diffraction (XRD). In conjunction with accelerated environmental tests, electrochemical corrosion testing methods including Zero Resistance Ammetry (ZRA), Cathodic Polarisation (CP) and Linear Polarisation Resistance (LPR) monitoring techniques were used in order to understand the mechanism of the corrosion protection and the effect of alloying elements. To correlate the effect of spray parameters on coating morphology, SEM, EDX and optical microscopy was used. The project ultimately aims to develop an understanding of the microstructure, composition and wear resistance of novel aluminium alloy coatings, relate this to the performance of the coatings in different environments and use this knowledge to optimise the alloy composition and spray parameters. References 1. Gudic.S, Smoljko.I, Kliskic.M, Journal of Alloys and Compounds 505, 2010, 54. 2. Carroll. W.M, Breslin. B.N, Corrosion Science 33(7), 1992, 1161 3. Mathiyarasu. J, Nehru. L.C, Subramaniam, P, Palaniswamy.N, Rengaswamy.N.S, Anti-Corrosion Methods and Materials 48 (5),2001, 324 4. Barbucci.A, Cerisola.G, Bruzzone.G, Saccone .A, Electrochimica Acta, 42(15), 1997,23695. Schweitzer A.P, 'Fundamentals of Metallic Corrosion', CRC Press 2007 (2nd ed.) 6. Krokhmal'nyj A.M, Fiziko-khimicheskaya mekhanika materialov, 29 ,1993, 37-417. Gabe .D.R, 'Principles of Metal Surface Treatment and Protection', Pergamon Press 1972 (1st ed.)